1. Field of the Invention
The present invention relates to a variable power projection lens for an optical projector such as for example a projection television.
2. Description of the Prior Art
Conventionally, there is known an optical projector for producing a large television picture by projecting a small television picture onto a screen at an enlarged size. In this type of projector, separate monochromatical B (blue), G (green) and R (red) images are projected onto the screen via three respective projection lenses, with the B, G and R images being superimposed on the screen so as to form a colored image.
Such projectors, however, have drawbacks in that, first, the need for three separate lenses for projecting B, G and R images results in a high cost and an ampler space for the optical system, and, second, it is very difficult to make the respective B, G and R images coincide.
To solve the problems, a projector has been used wherein dichroic prisms or dichroic mirrors combine light beams of B, G and R images, and then it is projected onto the screen using a single projection lens.
The projection lens of the projector therewith requires objective-sided telecentricity for prevention against color shading and a long back focus for inserting the prisms or mirrors into an optical path between the small television image and the projection lens.
Further, it necessitates a variable focal length for projecting an optionally magnified image onto an optionally sized screen which is separate from the projector.